1. Field of the Invention
The present invention relates to a method for fabricating a solid state image sensor, and more particularly, to a method for fabricating a solid state image sensor, which can improve a charge transfer efficiency of an end terminal.
2. Background of the Related Art
Referring to FIG. 1, a related art solid state image sensor is provided with a plurality of photodiodes (PD) 1 each for converting a light signal into an electrical video charge signal, vertical charge coupled devices (VCCD) 2 each for transporting a video charge converted by the photodiode 1 in a vertical direction, a horizontal charge coupled device (HCCD) 3 for transporting the video charge transported in the vertical direction by the VCCD's 2 in a horizontal direction, an end terminal inclusive of a sensing amplifier 4 for sensing the video signal charge transported in the horizontal direction by the HCCD 3.
A related art method for fabricating an end terminal of a solid state image sensor will be explained with reference to the attached drawings. FIG. 2 illustrates a layout of a related art end terminal of a solid state image sensor. FIG. 3 illustrates a section across line I--I in FIG. 2, and FIGS. 4a and 4b illustrate plan views showing the steps of a related art method for fabricating an end terminal of a solid state image sensor. FIG. 5 illustrates levels of potentials across line II--II in FIG. 4b, and FIG. 6 illustrates levels of potentials across line 111--111 in FIG. 4b.
Referring to FIGS. 2 and 3, the related art end terminal of a solid state image sensor is provided with a p type well 12 formed in a surface of an n type semiconductor substrate 11, a bulk charge coupled device (BCCD) 13 formed in a surface of the p type well 12, transfer gates 14 for the HCCD 3 formed on the semiconductor substrate with an insulating film in between, insulated from one another, an output gate (OG) 15 formed on the semiconductor substrate 11 with an insulating film in between on one side of the transfer gates 14 insulated therefrom, a reset gate 16 formed on the semiconductor substrate 11 with an insulating film in between on one side of the output gate 15 spaced therefrom, a heavily doped n type floating diffusion region (FD) 17 formed in the BCCD 13 between the output gate 15 and the reset gate 16 and connected to the sensing amplifier 4, and a heavily doped n type reset drain region 18 formed in a surface of the BCCD 13 on one side of the reset gate 16.
Referring to FIGS. 4a and 4b, a related art method for fabricating an end terminal of a solid state image sensor starts from coating a photoresist film 21 on a semiconductor substrate (not shown) in a state a p type well (not shown) is formed in a surface of the semiconductor substrate having an n type floating diffusion region 17 and a reset drain region 18 defined thereon and a BCCD is formed in a surface of the p type well. Subjecting the photoresist film 21 to selective exposure and development leaving a portion of the photoresist film 21 under which a floating diffusion region 17 is to be formed. The photoresist film 21, subjected to selective exposure and development, is used as mask in injecting p type impurity ions into an entire surface. As shown in FIG. 3, this p type impurity ion injection increases a concentration of the impurity only in a portion `H` of the BCCD 13 at which the floating diffusion region 17 is to be formed. In following steps, transfer gates 14, an output gate 15, a reset gate 16, floating diffusion region 17, and a reset drain region 18 are formed.
Levels of potentials in the related art end terminal of a solid state image sensor having p type impurity ions injected thereto are shown in FIG. 5, and in particular the even potential of output gate 15 can be noted. FIG. 6, shows the HCCD 3 having an even potential between a point `A` and a point `B`.
However, the related art method for fabricating a solid state image sensor has a problem in that the charge transfer efficiency of the end terminal of the solid state image sensor is low due to small variations of potential coming from the even potentials both in the output gate and a central portion of the HCCD. The problem is caused by the p type impurity ion injection increasing a concentration of impurities only in a portion H`at which the floating diffusion region is to be formed.